A heat sensitive recording material has been widely used as a recording material for a facsimile machine, a computer, another measuring machine and the like, because of such advantages as being maintenance-free, noiseless, low cost, etc.
It has now been required that the heat sensitive recording material has high heat-sensitivity or develops dark color with low energy, together with speeding up of transmittance of a facsimile machine and of printing out of a computer terminal. In order to accomplish the above requirement, many studies have been done, focusing on improving the heat sensitivity between a thermal head and a heat sensitive material. As the result, the following processes are proposed:
(a) A heat sensitive recording material is calendered to enhance its surface smoothness (see Japanese Patent Publication No. 20142/1977 and Japanese Laid-Open Publication No. 115255/1979).
(b) Paper is made by a cylinder paper machine and then cast-dried on one-side. The cast side is coated with a heat sensitive composition (see Japanese Laid-Open Publication No. 208297/1982).
(c) A base paper supporter of a heat sensitive paper is sized on the surface to prevent decline of the surface smoothness which occurs when coating a heat sensitive composition (see Japanese Laid-Open Publication No. 177281/1986).
If smoothness is enhanced by the calendar treatment, color density increases, but sticking is also increased therewith. Background coloring often occurs by the pressure in the calendar process. In the processes (b) and (c), it is difficult to constantly obtain heat sensitive paper having excellent properties, because its properties vary depending upon physical properties of the heat sensitive composition.
The present inventors have found the fact that the smoothness of the surface is lowered because a binder in the heat sensitive composition penetrates or spreads into paper matrix during coating and that the background coloring occurs because an amount of the binder left on the surface becomes insufficient to divide a dye and a developer. In order to prevent this penetration o spread of the binder into paper, so-called "binder migration", the present inventors intensely studied the problem and found that binder migration is effectively prevented by using an acrylic aqueous resin dispersion containing resin particles having a particle size of 0.001 to 0.05 microns which is prepared by a specific process. A heat sensitive paper which employs this aqueous resin dispersion can effectively prevent undesirable coloring before heating and has excellent color development sensitivity.